The present invention relates to a filter module such as multiplexer/demultiplexer module, used in an optical communication system of the dense wavelength division multiplexing (DWDM).
A typical optical communication system uses multiplexer modules and demultiplexer modules. A multiplexer module multiplexes (couples) two or more optical signals having different wavelengths and outputs the multiplexed signals to a single optical fiber. A demultiplexer module demultiplexes (divides) the wavelength-division-multiplex signals transmitted by optical fibers into separate signals.
FIG. 14 shows a prior art filter module used as modules discussed above. The filter module includes a single core optical fiber collimator (single fiber collimator) 21, a dual core optical fiber collimator (dual fiber collimator) 22, a cylindrical tube 23 for integrally holding the optical fiber collimators 21, 22. A filter 24 is joined to the end of the single fiber collimator 21 with adhesive. The filter 24 has wavelength selective property.
The single fiber collimator 21 includes a single core capillary 26, a gradient index rod lens 27, and a tube 28. The single core capillary 26 holds a single optical fiber 25. The tube 28 holds the single core capillary 26 and the rod lens 27. Referring to FIG. 14, the radiating end of the optical fiber 25 and the right end of the single core capillary 26 are ground to form a slant surface. An end of the rod lens 27 that faces the slant surface is also ground to form a slant surface. The single core capillary 26 and the rod lens 27 are aligned to match the axes and to accommodate angle difference, and then are fixed with adhesive in the tube 28 to maintain the aligned positions.
The dual fiber collimator 22 includes a double core capillary 32, a gradient index rod lens 33, and a tube 34. The double core capillary 32 holds two optical fibers 30, 31. The tube 34 integrally holds the double core capillary 32 and the rod lens 33. The facing ends of the double core capillary 32 and the rod lens 33 are ground to form slant surfaces. The double core capillary 32 and the rod lens 33 are adjusted to align the axes and accommodate angle difference. Thereafter, adhesive 35 is annularly applied to and placed between the slant surfaces to maintain the adjusted positions. The adhesive 35 acts to attach the double core capillary 32 and the rod 33 to each other. The joint is covered with a reinforcing adhesive 36. A short tube 37 is fitted about the right end of the double core capillary 32. The tube 34 is locate about the tube 37, the double core capillary 32, and the rod lens 33. Adhesive 38 fills the space that is inside the tube 34 and surrounds the double core capillary 32 and the rod lens 33. Accordingly, the double core capillary 32 and the rod lens 33 are held in the tube 34.
After forming the two optical fiber collimators 21, 22, the filter 24 is joined to the front end of the single fiber collimator 21, or the right end of the rod lens 27 with adhesive. Also, an optical device 39, such as a filter, is joined to the left end of the rod lens 33 with adhesive.
Thereafter, the two optical fiber collimators 21, 22 are aligned to match the axes and to accommodate angle difference and are joined to each other with adhesive 40 in the tube 23 so that the aligned positions are maintained. The filter module of FIG. 14 is thus manufactured.
However, since the above described prior art filter module has a great number of components, there are too many aligning and fixing procedures. This increases the time for manufacturing the filter module and lowers the reliability. Also, the manufacturing costs are increased.